Peter Zijlstra 119251855f x86/retpoline: Simplify retpolines ... Due to: c9c324dc22 ("objtool: Support stack layout changes in alternatives") it is now possible to simplify the retpolines. Currently our retpolines consist of 2 symbols: - __x86_indirect_thunk_\reg: the compiler target - __x86_retpoline_\reg: the actual retpoline. Both are consecutive in code and aligned such that for any one register they both live in the same cacheline: 0000000000000000 <__x86_indirect_thunk_rax>: 0: ff e0 jmpq *%rax 2: 90 nop 3: 90 nop 4: 90 nop 0000000000000005 <__x86_retpoline_rax>: 5: e8 07 00 00 00 callq 11 <__x86_retpoline_rax+0xc> a: f3 90 pause c: 0f ae e8 lfence f: eb f9 jmp a <__x86_retpoline_rax+0x5> 11: 48 89 04 24 mov %rax,(%rsp) 15: c3 retq 16: 66 2e 0f 1f 84 00 00 00 00 00 nopw %cs:0x0(%rax,%rax,1) The thunk is an alternative_2, where one option is a JMP to the retpoline. This was done so that objtool didn't need to deal with alternatives with stack ops. But that problem has been solved, so now it is possible to fold the entire retpoline into the alternative to simplify and consolidate unused bytes: 0000000000000000 <__x86_indirect_thunk_rax>: 0: ff e0 jmpq *%rax 2: 90 nop 3: 90 nop 4: 90 nop 5: 90 nop 6: 90 nop 7: 90 nop 8: 90 nop 9: 90 nop a: 90 nop b: 90 nop c: 90 nop d: 90 nop e: 90 nop f: 90 nop 10: 90 nop 11: 66 66 2e 0f 1f 84 00 00 00 00 00 data16 nopw %cs:0x0(%rax,%rax,1) 1c: 0f 1f 40 00 nopl 0x0(%rax) Notice that since the longest alternative sequence is now: 0: e8 07 00 00 00 callq c <.altinstr_replacement+0xc> 5: f3 90 pause 7: 0f ae e8 lfence a: eb f9 jmp 5 <.altinstr_replacement+0x5> c: 48 89 04 24 mov %rax,(%rsp) 10: c3 retq 17 bytes, we have 15 bytes NOP at the end of our 32 byte slot. (IOW, if we can shrink the retpoline by 1 byte we can pack it more densely). [ bp: Massage commit message. ] Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org> Signed-off-by: Borislav Petkov <bp@suse.de> Signed-off-by: Ingo Molnar <mingo@kernel.org> Link: https://lkml.kernel.org/r/20210326151259.506071949@infradead.org		2021-04-02 12:42:04 +02:00
..
accounting
arch	Merge branch 'x86/cpu' into WIP.x86/core, to merge the NOP changes & resolve a semantic conflict	2021-04-02 12:36:30 +02:00
bootconfig
bpf	Merge https://git.kernel.org/pub/scm/linux/kernel/git/bpf/bpf	2021-02-26 13:16:31 -08:00
build	perf build: Move feature cleanup under tools/build	2021-03-06 16:54:24 -03:00
cgroup
debugging
edid
firewire
firmware
gpio	tools: gpio: remove uAPI v1 code no longer used by selftests	2021-02-15 11:43:28 +01:00
hv
iio
include	Linux 5.12-rc5	2021-04-02 12:33:16 +02:00
io_uring
kvm/kvm_stat
laptop
leds
lib	libbpf: Fix BTF dump of pointer-to-array-of-struct	2021-03-19 14:14:44 -07:00
memory-model
objtool	x86/retpoline: Simplify retpolines	2021-04-02 12:42:04 +02:00
pci
pcmcia
perf	Linux 5.12-rc5	2021-04-02 12:33:16 +02:00
power	platform-drivers-x86 for v5.12-1	2021-02-22 08:50:01 -08:00
scripts	New features:	2021-02-22 13:59:43 -08:00
spi
testing	Merge branch 'akpm' (patches from Andrew)	2021-03-25 11:43:43 -07:00
thermal/tmon
time
tracing	tracing/tools: fix a couple of spelling mistakes	2021-02-25 12:54:16 -05:00
usb
virtio
vm
wmi
Makefile	tracing/tools: Add the latency-collector to tools directory	2021-02-12 11:52:59 -05:00